spectrum analyzer

Do you have an EMC probe in your toolkit? Probably not, unless you’re in the business of electromagnetic compatibility testing or getting a product ready for the regulatory compliance process. Usually such probes are used in anechoic chambers and connected to sophisticated gear like spectrum analyzers – expensive stuff. But there are ways to probe the electromagnetic mysteries of your projects on the cheap, as this DIY EMC testing setup proves.

As with many projects, [dimtass]’ build was inspired by a video over on EEVblog, where [Dave] made a simple EMC probe from a length of semi-rigid coax cable. At $10, it’s a cheap solution, but lacking a spectrum analyzer like the one that [Dave] plugged his cheap probe into, [dimtass] went a different way. With the homemade probe plugged into an RTL-SDR dongle and SDR# running on a PC, [dimtass] was able to get a decent approximation of a spectrum analyzer, at least when tested against a 10-MHz oven-controlled crystal oscillator. It’s not the same thing as a dedicated spectrum analyzer – limited bandwidth, higher noise, and not calibrated – but it works well enough, and as [dimtass] points out, infinitely hackable through the SDR# API. The probe even works decently when plugged right into a DSO with the FFT function running.

Again, neither of these setups is a substitute for proper EMC testing, but it’ll probably do for the home gamer. If you want to check out the lengths the pros go through to make sure their products don’t spew signals, check out [Jenny]’s overview of the EMC testing process.

It used to be a spectrum analyzer was an exotic piece of gear. However, these days it is pretty common for a scope to have some ability to do the job — that is, plot amplitude versus frequency. However, a dedicated commercial product will usually have a lot more bandwidth and other features. [Signal Path] picked up an Anrtitsu 7.1 GHz portable spectrum analyzer. An expensive bit of kit — anywhere from around $4,000 to $8,000 on eBay — if it is working, but this one was not. It needed power, but it was also missing the internal flash card that the device uses to boot.

Being portable, there’s a lot of digital and RF electronics crammed into a very small space. The initial tear down didn’t look very interesting because it was mostly an RF shield. However, many tiny screws later, you can finally see the actual electronics.

Normally, non-contact tachometers are optically coupled, using photoreceptors to measure light flashing off of a shaft or a tool. But that requires a clear view of the machine, often putting hands far too close to the danger zone. [Matthias Wandel]’s method doesn’t require line of sight because it relies on a cheap spectrum analyzer app to listen to a machine’s sound. The software displays peaks at various frequencies, and with a little analysis and some simple math, the shaft speed of the machine can be determined. [Matthias] explains how to exclude harmonics, where to find power line hum, isolating commutator artifacts, and how to do all the calculations. You’ll need to know a little about your tooling to get the right RPM, and obviously you’ll be limited by the audio frequency response of your phone or tablet. But we think this is a great tip.

The eBay addiction starts small. One night you’re buying $3 buck-boost converters and cheap Chinese USB power packs. The next thing you know you’re spending thousands on dead instruments with no documentation. You’ve got the skills though, and if your bet that you can diagnose and repair a 14 GHz real-time spectrum analyzer is right, you’ll be putting a snazzy instrument on the bench for a fraction of the original $50,000 it cost.

Make some popcorn and get cozy before settling in to watch [Shahriar]’s video below, because it clocks in at just over an hour. But it’s pretty entertaining, and just seeing how Tektronix built the RSA 6114A spectrum analyzer is worth the time. Things are different when you’re piping microwave signals around the chassis of a beast such a this, the interior of which is densely packed with pluggable modules. Tek factory service would no doubt perform a simple module swap to get this machine running again, but [Shahriar] wasn’t having any of that on his $2,700 eBay find. After isolating the problem to the local-oscillator generator module, [Shahriar] takes us on a tour of where the signals go and what they do. We won’t reveal the eventual culprit, but suffice it to say that after a little SMD rework, [Shahriar] has a very fancy new instrument for the shop.

If you buy an amateur transceiver cheap enough to make a reasonable grab bag gift or stocking stuffer, you get what you pay for. And if this extensive analysis of cheap radios is any indication, you get a little more than you pay for in the spurious emissions department.

Amateur radio in the United States is regulated by the FCC’s Part 97 rules with special attention given to transmitter technical specifications in Subpart D. Spurious emissions need to be well below the mean power of the fundamental frequency of the transmitter, and [Megas3300] suspected that the readily available Baofeng UV-5RA dual-band transceiver was a little off spec. He put the $20 radio through a battery of tests using equipment that easily cost two orders of magnitude more than the test subject. Power output was verified with a wattmeter, proper attenuators were selected, and the output signal scanned with a spectrum analyzer. Careful measurements showed that some or all of the Baofeng’s harmonics were well above the FCC limits. [Megas3300] tested a few other radios that turned out to be mostly compliant, but however it all turned out, the test procedure is well documented and informative, and well worth a look.

The intended market for these radios is more the unlicensed crowd than the compliant ham, so it’s not surprising that they’d be out of spec. A ham might want to bring these rigs back into compliance with a low pass filter, for which purpose the RF Biscuit might prove useful.

The “normal” way to build a spectrum analyzer is to collect a bunch of samples and run a Fast Fourier Transform (FFT) on them all in one shot. As the name implies, the FFT is fast, and the result is the frequency components of the sampled data. [agp.cooper]’s “wrong” way to do it takes the Goertzel algorithm, which is used for detecting the intensity of a particular frequency, and scanning across the frequency range of interest. It’s a lot slower than a single FFT but, importantly for the ATtiny85 that he implements this on, it’s less demanding of the RAM.

Shortwave listening has always been a mainly nocturnal hobby. To get the real DX, one had to wait for favorable ionospheric conditions after sunset and spend hours twisting knobs while straining to pick voices from half a planet away out of the noise. But who has time for that in today’s world? And what of the poor city-dwelling SWL, with antenna limitations and often elevated noise floor in the urban jungle?Continue reading “Cache Shortwave Signals for Later with this SDR Spectrum Grabber”→